You can still access the members area via
https://members.vbulletin.com
It appears to have only been the forums, the main home page and all subsequently related pages work along with the members area as well.
If your vB5 forum is hacked, close the board and await an announcement and new security patch because currently since we (the public) do not know the exploit used, overwriting with fresh files from a version with an apparent exploit won't be of much use.
*Although to be fair this could have been some other form of exploit... server possibly however I doubt that based on it only being the vB5 powered forums down currently - we honestly do not know yet

. *This may also be something that affected the specific version on vbulletin.com, meaning that usually they run a slightly newer version than what is currently released so it's entirely possible only their version was compromised and no others are at risk however that's dependent on a number of factors.
Is it bad? Well sure no one likes being defaced that way but let's not start a panic and have chaos ensue in this thread, we'll know more soon

.